In the prior art, polyvinyl chloride (PVC) plastisols which comprise a plasticizer having PVC particles dispersed therein and combine low temperature baking property and storage stability and have low-temperature flexibility, have been greatly used as a body sealer in automotive assembly line. However, these PVC plastisols had disadvantages that since the main component is PVC, a generation of hydrogen chloride gas may bring a destruction of ozone layer, a cause of acid rain and a cause of dioxin.
So, it has been strongly wanted that such PVC plastisols are replaced by acrylic plastisols which comprise a plasticizer having acrylic resin particles and a filler dispersed therein. This acrylic plastisol has no problem for coping with both low temperature baking property and storage stability and moreover is excellent in workability and it can be used in any existing facilities, and hence, it is very promising material, but it is bad particularly in low-temperature flexibility and is very expensive.
On the other hand a one-pack type thermosetting polyurethane sealer is currently employed on trial which comprises a curing agent and a blocked urethane prepolymer wherein free NCO groups of a terminal NCO-containing urethane prepolymer obtained by reaction of polyol with an excess amount of polyisocyanate compound have been blocked. Although such a sealer has excellently enhanced physical properties, strength and elongation at cold, it was expensive and bad in workability, and furthermore, it was difficult to cope with both storage stability and low temperature baking property using such a sealer. For example, if low temperature baking property is regarded at as important, a blocking agent of low dissociation temperature in the blocked polyurethane prepolymer will result in reduced storage stability. In contrast, when a blocking agent of high dissociation temperature is used, desirable physical properties can not be developed unless it is heated typically at a temperature of 160° C. or higher, and finally low temperature baking property becomes inferior.